The objective of the present study is to determine if the widely used drug of abuse marihuana, has the capacity to produce genetic damage in the form of chromosome breaks. A twofold approach is being used: First, blood samples for leukocyte culture obtained from healthy human volunteer subjects both before and after oral application of a variety of doses of marihuana extract and synthetically produced delta-9-THC are used to make metaphase chromosome preparations which are analyzed for chromosome breakage. In addition, direct mitotic chromosome preparations are being made from rat bone marrow and testis after intragastric applications of a series of doses of marihuana extract and delta-9-tetrahydrocannabinol, and observations are being made on the frequency of chromosome breaks for comparison with sham-treated negative controls and Mitomycin-C-treated positive controls. The second portion of this study permits the use of a maximum tolerated dose and an intermediate dose as well as an equivalent use-level application of drug.